1. Field of the Invention
The invention in general relates to automobile brake switches and more particularly to such switches that activate a vacuum driven system.
2. Description of the Prior Art.
Switches that are activated by the brake mechanism of an automobile are well known in the art. Such switches generally comprise a housing which includes a switch chamber and a cylindrical plunger chamber, a plurality of electrical switches in the switch chamber, and a plunger movable in the plunger chamber. The plunger is driven by the brake mechanism to operate the switches. See, for example, U.S. Pat. No. 4,384,176 issued on an invention of Benjamin F. Chestnut. In the above patent, the switches are electrical switches. It is also known to use the plunger to activate a vacuum driven system. Such switches operated to open and close a vent to a vacuum line. Generally an automobile brake system will include both electrical and vacuum switches since the vacuum switch offers a mechanical alternative that can operate if the electrical system fails. The nature of the automobile brake system requires that a brake switch be activated at some point along the travel of the plunger, rather than just at the extremes of travel. Thus the switch must be designed to permit the switch to be held on or off during a portion of the travel of the plunger. Prior art vacuum switches have accomplished this by venting the vacuum line at the bottom of a deep cylinder. A plunger rod carries an o-ring in a direction along the axis of the cylinder, and the vacuum is sealed and vented by the o-ring entering or exiting the cylinder. After the o-ring enters the cylinder to seal the vent, the plunger can continue to move and at the same time continue to seal the vent by driving the o-ring further down the cylinder. Similarly after opening the vent by exiting the cylinder, the plunger can continue to vent the tube by moving the o-ring further from the cylinder. While such vacuum switches are effective, the fact that the o-ring continues to move in the cylinder after the vent is sealed leads to wear of the o-ring, decreased reliability, and difficulty in achieving long lifetimes with such switches.